For a number of years it has been known to use the transit time of sound waves for distance measuring purposes. Echo ranging techniques including the commonly known sonar, have been known in the art for a number of years.
More recently, one way acoustic distance measuring devices have been provided which use a radio frequency signal both to activate a counter at an acoustic receiving station and to activate an acoustic transmitter at a transmitting station. Such a device is shown in U.S. Pat. No. 4,055,830 to Wilson et al. The apparatus of the Wilson patent uses a radio signal to simultaneously start a counter and begin transmission of an acoustic signal. Upon receipt of the signal by a microphonic device, the signal is amplified through a multistage amplifier having a constant amplitude output which is fed to an active filter. The output of the active filter is rectified and applied to the input of a differentiator, the output of which gates off the counter. The disclosure of the Wilson patent acknowledges that the group delay of an active filter is a function of input amplitide and compensates for possible time skew in the measurements by providing the constant amplitude amplifier.
However, the apparatus of the Wilson does not take into effect possible variations from unit to unit in group delay of an active filter manufactured from off-the-shelf components, nor does the Wilson apparatus take into account the possible differences in time domain responses of differentiators such as those used in the Wilson control circuitry.
The basic principles of acoustic distance measuring are known. In the art of acoustic distance measuring, a major limitation of accuracy of the apparatus is both the absolute time delay of frequency sensitive circuitry and the variability of time response of such circuitry, particularly rise times of electrical outputs in response to initial excitations.
Furthermore, prior art acoustic measuring devices are started and stopped upon initial detection of a particular tone transmitted. This aggravates the aforementioned problem of consistency from device to device or calibration of a particular device according to the group delay and rise time responses of the components used.
The desirability of using frequencies of rather long acoustic wavelength within the audible frequency range is known in the art. However, prior art apparatus such as that of the Wilson patent has limited capability to work in environments in which any substantial amount of ambient noise may be encountered which has a frequency content in the range of the transmitted signal. Such ambient noise would tend to falsely trigger the stop function of the Wilson patent since this stop function is responsive to the derivitive of the rectified output of a notch filter.
It will therefore be appreciated that it is desirable to provide acoustic measuring apparatus which will function satisfactorily in the presence of ambient noise and which can use broader band filters in the signal processing path than the high Q filters used in single tone systems.
It is further desirable to provide an acoustic distance measuring apparatus which may be inexpensively constructed from available components having conventional ranges of value and parameter tolerances wherein the device will not have to be individually calibrated to compensate for characteristics of the individual devices used.